Cellular and Molecular Basis of Inheritance Flashcards
Describe the structure of a chromosome
2 identical chromatids
Joined at the centromere
Short P arms
Long Q arms
What is chromatin made of?
DNA + RNA + Proteins
How is DNA packaged?
DNA winds around histone proteins, forming nucleosomes
Nucleosomes are organised into solenoids (tightly packed helix)
Solenoids fold into loops
What is heterochromatin vs euchromatin?
Heterochromatin = tightly packaged DNA, not being expressed Euchromatin = unwound DNA, available for expression
What percentage of DNA is actual genes?
2%
What makes up the rest of DNA that is not genes?
Repetitive DNA that controls gene regulation TATA Box 5' UTR 3' UTR Introns Translation initiation codon Translation termination codon PolyA tail region
What is the function of the TATA Box?
It is a promotor region, that indicates where DNA transcription should start
What is the function of the PolyA tail?
Protects mRNA from degradation by phosphatases and nucleases
What is the PolyA tail?
Tail of around 200 A nucleotides
Added onto mRNA transcript before it leaves the nucleus
What does intragenic mean?
Introns within genes
What does intergenic mean?
Non coding regions between genes
What is MtDNA and what are it’s features?
Mitochondrial DNA Circular Double stranded Inherited maternally 37 genes
What happens during prophase?
The chromatin condenses into visible chromosomes )(
The nuclear envelope and organelles disappear
The spindle forms
What happens during metaphase?
Chromosomes connect to the spindles
The chromosomes align along the metaphase plate in the centre of the cell
What happens during anaphase?
The spindles contract and the centromere splits
One chromatid from each chromosome is pulled to opposite poles of the cell )—(
What happens during telophase?
The chromosomes cluster at the poles
A nuclear envelope reforms around each group
Organelles reform
How many chromosomes in a diploid cell vs a haploid cell?
Diploid = 46 chromosomes (23 pairs) Haploid = 23 chromosomes
What occurs during meiosis I?
DNA replicates and homologous chromosomes pair up: )( )(
Genetic variation is introduced so that each chromosome contains non-identical chromosomes
Chromosomes pairs are separated )( — )(
What occurs during meoisis II?
No DNA replication
Chromosomes are separated at the centromere to form 4 haploid cells
When is genetic variation introduced during meiosis?
Crossing over occurs during prophase I
Independent assortment occurs during metaphase I
Give an example of a genetic disorder caused by meiosis going wrong
Trisomy
Occurs when chromosomes do not split evenly during meosis I or meosis II
Downs Syndrome is trisomy of chromosome 21
Which 2 types of cells do not undergo mitosis?
Neurons and muscle cells
What happens during G1 phase?
Cellular contents duplicated, excluding DNA
What occurs during S phase?
DNA replication of all 46 chromosomes
What happens during G2 phase?
Checks that all DNA is replicated successfully
Preparation for mitosis
Any DNA repair that is required
What is checked during G1/S phase?
Is the environment favourable?
Presence of growth factors?
Is there enough nutrients?
What is checked during G2/M phase?
Is all DNA replicated?
Is all DNA damage repaired?
What is checked during the checkpoint within M phase?
Are all chromosomes attached to the mitotic spindle?
What are tumour suppressor genes?
Genes that produce proteins that inhibit cell proliferation
Loss of function mutations in these genes lead to uncontrolled cell proliferation
What are proto-oncogenes?
Genes that produce proteins that stimulate cell proliferation
Gain of function mutations in these genes cause it to become an oncogene - lead to uncontrolled cell proliferation
Example of a tumour suppressor gene
Retinoblastoma (Rb) gene
RB1
Inhibits E2F transcription factor, which transcribes genes needed for S phase
Rb can be inhibited by phosphorylation by CDK4 + CDK6
Loss of function mutations in Rb can lead to retinoblastoma, which is cancer of the retina, which occurs almost exclusively in young children
What is apoptosis?
Programmed cell death
Provides health benefit
What are some potential uses of apoptosis?
Shaping tissues e.g. between fingers and toes
Killing off ineffective T cells
What can be some detrimental consequences of apoptosis?
Uncontrolled apoptosis can lead to Alzheimers (degeneration of neurons) and Parkinsons (loss of dopinamergic neurons)
What is necrosis?
Cells die by lysis
Unplanned cell death due to external factors such as trauma, infection, toxins, radiation, heat
What are the main differences between the processes of apoptosis and necrosis?
Apoptosis occurs to a single cell, necrosis occurs to a group of cells
During apoptosis, cells shrink. During necrosis, cells swell and lyse
Apoptosis is ATP dependent, necrosis is passive
During apoptosis, lysosomes remain intact. During necrosis, lysosomes leak their contents
What is a cancer syndrome?
Inherited genetic mutations predispose an individual to developing cancer
Not guaranteed to develop cancer but much more likely
Much more likely to cause early onset cancer
Mutations in which 3 types of genes are most often the cause of cancer syndromes?
Tumour suppressor genes
Proto-Oncogenes
DNA repair genes
What is the pattern of inheritance for cancer syndromes?
Usually autosomal dominant, as only one faulty allele is needed for the predisposition
Sometimes autosomal recessive if two faulty alleles are needed for predisposition
What is Knudson’s two-hit hypothesis?
Cancer is the result of an accumulation of DNA mutations, usually in proto-oncogenes or tumour suppressor genes
As these genes are heterozygous, both alleles need to be ‘hit’ to cause cancer
If the first mutation is inherited, only one more mutation is needed, so you are more likely to develop cancer younger
What is hereditary cancer vs sporadic cancer?
Hereditary = One allele inherited already mutated Sporadic = both mutations acquired randomly
What is the function of the p53 gene?
Tumour suppressor
p53 halts progression into S phase when it detects DNA damage
The cell is given time to repair the DNA
If the DNA cannot be repaired, the cell will apoptose
What is Li-fraumeni syndrome?
Inherited mutation in one allele for the p53 gene
Increased risk of osteosarcoma, breast cancer, leukaemia etc
Inherited autosomal dominantly
How does p53 work?
DNA damage produces ATM Kinase
Phosphorylation of p53 (so it cannot be ubiquitylated)
p53 activates expression of genes including p21
p21 inhibits CDKs
CDKs cannot phosphorylate Rb
Rb free to inhibit E2F
No progression into S phase
What is hereditary breast-ovarian cancer syndrome?
Autosomal dominant cancer syndrome
Mutations in BRCA1 or BRCA2 genes, which are DNA repair genes, involved in repairing double stranded breaks
DNA damage is not repaired correctly, leading to faster accumulation of mutations
Predisposes to breast, ovarian, fallopian tube carcinoma and prostate cancer in men
How can cancer syndromes be tested for?
Take a sample of body fluid or tissue (blood, saliva, amniotic fluid)
Use NGS to determine DNA sequence of mutation
Or use PCR to look for a specific mutation
What is familial adenomatous polyposis?
Autosomal dominant cancer syndrome
Many polyps form in the epithelium of the large intestine
Start benign, may transform into malignant cancer if left untreated
Usually adult onset
What are the 3 types of familial adenomatous polyposis?
FAP
Attenuated FAP
Autosomal recessive FAP
What is ‘normal’ FAP?
Mutation in APC gene leads to total loss of function. APC combines with axin and a kinase to form part of the beta-catenin destruction complex in the Wnt signalling pathway. Beta-catenin is a transcription factor for cell proliferation.
93% will develop cancer by age 50
What is attenuated FAP?
A mutation in the APC gene that leads to production of an attenuated APC protein. Still functional but has impaired function.
What is autosomal recessive FAP?
Mutation in the MUTYH gene. Has much milder effects, and required a mutation to be inherited from both parents
What are the symptoms of FAP?
Blood in stool
Alternating diarrhoea and constipation
Unexplained weight loss
Abdominal pain
What might lead to delayed diagnosis of FAP?
The symptoms are very similar to coeliac
What are the 4 main cyclins that control the cell cycle (in order of when their concentrations peak)
Cyclin D - present throughout cycle
Cyclin E - peaks at G1/S
Cyclin A - peaks in G2
Cyclin B - peaks at G2/M
Name 4 tumour suppressor genes
Retinoblastoma
p53
BRCA1/BRCA2
APC
Name 3 proto-oncogenes
Myc/c-Myc
HER-2
MDM2
What does Myc do?
Myc is a transcription factor involved in many functions (cell proliferation, apoptosis, cellular transformation)
Involved in Wnt, Hedgehog and MAPK signalling pathways
Enhanced by HIF-2a which is active in hypoxic conditions e.g. in the centre of tumours
What is HER-2?
-what does it stand for?
Human Epidermal growth factor Receptor
Codes for a tyrosine kinase receptor involved in MAPK and JAK/STAT signalling pathways
Overexpression of receptor causes dimerisation even when no ligand is bound
Has a role in the development of breast cancer
What is MDM2?
In normoxic conditions, MDM2 ubiquitylates p53 to mark it for degradation
Mutations in MDM2 lead to constitutive cell proliferation
What is a synonymous mutation?
The resulting amino acid sequence is not modified
What is a non-synonymous mutation, and the two types?
The resulting amino acid sequence is modified
Missense = mutation results in a codon for a different amino acid
Nonsense = mutations results in a premature stop codon, the protein product is non-functional
What is a carcinogen?
Promotes initiation of cancer by damaging the genome or disrupting cellular processes
What is metastasis?
Cells of a tumour break off and circulate through the lymph system or bloodstream
The start proliferation in a new location
Tumours disrupt function of organs
Describe the process of the MAPK cascade
Ligand (EGF, VEGF, IGF) binds to tyrosine kinase receptor
Dimerisation and trans-autophosphorylation
Recruits SH2 domain of Grb2
Grb2 brings SOS
SOS catalyses exchange of GDP for GTP on Ras
Ras activates MAP3K, which phosphorylates MAP2K, which phosphorylates MAPK
MAPK activates Myc
Myc transcribes genes for cell proliferation
Describe the process of the Wnt signalling pathway, when the ligand is present
Wnt binds to Frizzled receptor
Activation of dishevelled protein
Dishevelled inhibits the kinase in the beta-catenin complex
No destruction of beta-catenin
Describe the process of the Wnt signalling pathway, when the ligand is not present
The beta-catenin destruction complex forms, made of APC, Kinase and Axin
Destruction of beta-catenin
No cell proliferation
What is cellular senescence?
Cells lose their ability to divide due to ageing
Due to telomere shortening
Detected as DNA damage, which activates p53
Why are cancer syndromes considered dominant conditions, even though you need mutations in both alleles to cause cancer?
A Cancer syndrome is only a predisposition
You only need one mutated allele for a predisposition
What is hyperplasia?
Enlargement of an organ or tissue to to increased rate in cell proliferation
What is metaplasia?
The change of one type of differentiated cell into another
What is dysplasia?
Presence of abnormal cell types within a tissue
May be a pre-indication of cancer
What is neoplasia?
The presence of a new growth of tissue, due to abnormal cell proliferation
Give two examples of genes that beta-catenin transcribes
Cyclin D
Myc
